Pressure-tube reactor with pressurized moderator

ABSTRACT

A nuclear reactor can include a pressure vessel for containing a pressurized moderator at a first pressure. The nuclear reactor can also include a plurality of fuel channels for a coolant fluid at a second pressure. The plurality of fuel channels are fluidly connected at inlet ends thereof to a coolant supply conduit and are adapted to receive nuclear fuel bundles and to be mounted within the pressure vessel and surrounded by the moderator. The outlet ends of the fuel channels are fluidly connected to a coolant outlet conduit to enable the coolant fluid to circulate from the coolant supply conduit through the fuel channels to the coolant outlet conduit. The plurality of fuel channels maintain separation between the coolant fluid circulating within the fuel channels and the moderator.

This application is a National Stage of International Application No.PCT/CA2011/000459, filed Apr. 21, 2011, which claims the benefit ofProvisional Application No. 61/327,502 filed Apr. 23, 2010, each ofthose applications being incorporated herein in their entirety byreference.

FIELD

This invention relates to nuclear reactors, and more particularly tonuclear reactors having a pressure vessel to contain a pressurizedmoderator and separate pressurized coolant flowing through pressuretubes.

INTRODUCTION

Commercial nuclear power plants are known. Based on the mechanicaldesign of the pressure retaining components of the reactor core,commercial nuclear reactors can be grouped as either “pressure-vessel”or “pressure-tube” type reactors.

Examples of a pressure-vessel type reactor are Pressurized WaterReactors (PWR) and Boiling Water Reactors (BWR). In these reactordesigns nuclear fuel is contained in a large pressure vessel. In suchpressure-vessel type reactors the coolant and the moderator fluid may bethe same fluid and thus there is no need to maintain two differentfluids separated from one another within the pressure vessel. The singlefluid can be supplied to the pressure vessel using an inlet plenum andwithdrawn from the vessel using an outlet plenum. In such designs thereis no need to isolate a coolant fluid from a separate or differentmoderator fluid, consequently the plenums need not feed a plurality ofseparate, sealed fuel channels. The shared moderator/coolant fluid insuch reactors is typically light water (H₂O).

In one example, referred to as Pressurized Heavy Water Reactors (PHWR),a low pressure heavy water moderator surrounds the fuel channels and aseparate pressurized flow of heavy water coolant is circulated throughthe fuel channels. Examples of this type of reactor can be operatedusing natural uranium fuel. In these examples, the term “PressurizedHeavy Water” refers to the coolant in the fuel channels, not theseparate heavy water moderator.

In another example, a low pressure heavy water moderator fluid surroundsthe fuel channels and a separate pressurized flow of light water coolantis circulated through the fuel channels. This type of reactor can beoperated using enriched uranium fuel.

Traditional, horizontal pressure-tube nuclear reactors are known.Existing pressure-tube reactors include a plurality of individual fuelchannels or pressure tubes extending horizontally through a low pressurecalandria vessel containing a heavy water moderator. Nuclear fuelbundles are placed within the pressure tubes and a coolant fluid iscirculated through the pressure tubes to be heated by the nuclearreactions.

Coolant feeder pipes (coolant inlet pipes and coolant outlet pipes) inexisting pressure-tube reactors are an integral part of the circulatingheat transport system, connecting the in-reactor fuel channels with theprimary heat transport pipes. The low pressure calandria vessel commonlyhas separate calandria tubes, to define the space for the moderator, andthe pressure tubes extend through the calandria tubes. Garter springsmaintain spacing between each pair of a calandria tube and a pressuretube, and define an annulus.

A typical pressure-tube design can include a plurality of fuel channelsand twice as many feeder pipes (each tube having a corresponding inletand outlet feeder).

A feature of some existing pressure-tube designs is the on-line fuellingcapability. The use of separate feeders allows on-line fuelling througha removable end channel seal closure and a remotely operated fuellingmachine.

Existing reactor designs, both of the pressure-vessel and pressure-tubetype, cannot readily be adapted for use with a supercritical fluid, e.g.water, as the coolant and heat transfer medium. For a supercriticalfluid, this specification and the present invention propose aconfiguration of pressure tubes, suitable for use with such a fluid. Toreach a supercritical state, the coolant fluid will be maintained athigh pressures (for example at pressures greater than 23 MPa) and atelevated temperatures. Existing pressure tube and pressure-vessel typedesigns cannot withstand such high pressures, and existing pressure tubeand pressure vessel materials can be prone to increased corrosion andwear when exposed to supercritical fluids. Simply increasing the size orthickness of existing pressure tubes and pressure vessels may not bepossible due to manufacturing limitations or tube spacing requirementsand may affect reactor efficiency.

Creating pressure vessels for existing reactor designs to withstand highpressures and correspondingly high temperatures can be costly anddifficult, and exposure to supercritical fluid flows can erode exposedportions of the pressure vessel walls, which may lead to increasedmaintenance and premature failure.

SUMMARY

This summary is intended to introduce the reader to the more detaileddescription that follows and not to limit or define any claimed or asyet unclaimed invention. One or more inventions may reside in anycombination or sub-combination of the elements or process stepsdisclosed in any part of this document including its claims and figures.

According to one broad aspect of the invention, a nuclear reactorcomprising includes a pressure vessel for containing a pressurizedmoderator at a first pressure. The nuclear reactor also includes aplurality of fuel channels for a coolant fluid at a second pressure. Theplurality of fuel channels are fluidly connected at inlet ends thereofto a coolant supply conduit and are adapted to receive nuclear fuelbundles and to be mounted within the pressure vessel and surrounded bythe moderator. The outlet ends of the fuel channels are fluidlyconnected to a coolant outlet conduit to enable the coolant fluid tocirculate from the coolant supply conduit through the fuel channels tothe coolant outlet conduit. The plurality of fuel channels maintainseparation between the coolant fluid circulating within the fuelchannels and the moderator.

According to another broad aspect, a method of operating a nuclearreactor includes the steps of a) providing a pressure vessel containinga pressurized moderator; b) providing a plurality of fuel channelsextending through the pressure vessel, surrounded by the moderator; c)placing at least one nuclear fuel bundle within each fuel channel; d)circulating a coolant fluid through each fuel channel to be heated bythe nuclear fuel bundle contained therein, the coolant fluid beingseparate from the moderator; and e) extracting the coolant fluid fromeach outer conduit, without direct mixing the coolant fluid with themoderator and channeling the coolant fluid for further processing.

According to another broad aspect, a method of batch-refueling a nuclearreactor includes the steps of a) providing a pressure vessel to containa pressurized moderator and providing a plurality of outer conduitssurrounded by the moderator; b) providing a first core module detachablycoupled to the pressure vessel, the first core module comprising aplurality of first fuel bundles suspended within outer conduits; c)detaching the entire core module from the pressure vessel tosimultaneously extract the plurality of first fuel bundles; d) providinga second core module comprising a plurality second fuel bundles; and e)coupling the second core module to the pressure vessel so that theplurality of second fuel bundles are simultaneously received withincorresponding ones of the plurality of outer conduits.

According to another broad aspect, a nuclear reactor includes a pressurevessel defining a chamber for a moderator and a fueling tubesheet withinthe pressure vessel separating the moderator chamber from a plenumchamber for a coolant fluid. The nuclear reactor also includes an outletplenum for the coolant fluid within the plenum chamber. An inlet plenumis defined between the outlet plenum and the plenum chamber. The plenumchamber has at least one coolant outlet port extending through thepressure vessel to an exterior of the pressure vessel. The nuclearreactor also includes a plurality of closed ended pressure tubes mountedto the fueling tubesheet and extending into the moderator. The nuclearreactor also includes a plurality of fuel liners mounted to the outletplenum, for receiving fuel bundles. The coolant fluid can flow from theinlet plenum through the pressure tubes to the closed ends thereof, andthrough the fuel liners to the outlet plenum. The moderator ismaintained at a first pressure and the coolant fluid is maintained at asecond higher pressure. The fueling tubesheet and the pressure tubesmaintaining the difference between the first and second pressures.

DRAWINGS

For a better understanding of the pressure-tube reactor with separatepressurized moderator described herein and to show more clearly how theymay be carried into effect, reference will now be made, by way ofexample only, to the accompanying drawings which show at least oneexemplary embodiment, and in which:

FIG. 1 is a partial sectional, front plan view of an example of anuclear reactor;

FIG. 2 is a sectional view of a fuel channel that can be used in thenuclear reactor of FIG. 1;

FIG. 2A is an enlarged view of section 2A indicated on FIG. 2;

FIG. 3 is a partial sectional, isometric view of the reactor of FIG. 1;

FIG. 4 is a partially exploded view of the reactor shown in FIG. 3;

FIG. 5 is a sectional view of a connection of an outlet plenum used inthe reactor of FIG. 1;

FIG. 6 is a schematic view of a reactor having two core modules.

For simplicity and clarity of illustration, elements shown in thefigures have not necessarily been drawn to scale. For example, thedimensions of some of the elements may be exaggerated relative to otherelements for clarity. Further, where considered appropriate, referencenumerals may be repeated among the figures to indicate corresponding oranalogous elements.

DETAILED DESCRIPTION

Various apparatuses or processes will be described below to provide anexample of an embodiment of each claimed invention. No embodimentdescribed below limits any claimed invention and any claimed inventionmay cover processes or apparatuses that are not described below. Theclaimed inventions are not limited to apparatuses or processes havingall of the features of any one apparatus or process described below orto features common to multiple or all of the apparatuses describedbelow. It is possible that an apparatus or process described below isnot an embodiment of any claimed invention. Any invention disclosed inan apparatus or process described below that is not claimed in thisdocument may be the subject matter of another protective instrument, forexample, a continuing patent application, and the applicants, inventorsor owners do not intend to abandon, disclaim or dedicate to the publicany such invention by its disclosure in this document.

This specification generally describes a nuclear reactor that includes aplurality of pressurized fuel channels surrounded by a pressurizedmoderator. The reactor includes a pressure vessel for containing thepressurized moderator and a plurality of sealed pressure tubes forcontaining a flow of pressurized coolant, maintaining a separationbetween the coolant and the moderator fluids. An inlet plenum providescoolant to each of the fuel channels and an outlet plenum collects theheated coolant at the outlet of each fuel channel. The coolant fluid canbe light water or heavy water or any other suitable coolant fluid knownin the art. In some examples the coolant is in a supercritical conditionas it exits the fuel channels. It is to be understood that the presentinvention may be generally applicable to any reactor having both apressurized moderator (a pressure vessel) and separate coolant.

Referring to FIGS. 1, 3 and 4, one example of a nuclear reactor 100includes a plurality of pressurized fuel channels, for example fuelchannels 102 contained within a pressurized containment vessel orpressure vessel, for example reactor pressure vessel 104, that containsa moderator fluid in the calandria 105 a. For clarity, only a singlefuel channel is illustrated, but it is understood that the pressurevessel may contain and will usually contain a plurality of fuelchannels. Each fuel channel, 102 can be sized to accommodate a fuelliner tube 136, which further accommodates nuclear fuel rods orelements, shown schematically as fuel bundle/assembly 106 and can havean inner diameter, for example diameter 137 (FIG. 2A) between 2-25 cm,and in some examples between 5-15 cm. The fuel channels 102 canwithstand the expecting operating temperatures and pressures of thenuclear reactor 100 (although they may be designed to yield duringabnormal or emergency conditions), have suitable neutron absorptioncharacteristics (as explained in more detail below) and include a fuelbundle holder or fuel holding apparatus (not shown) that is adapted toreceive one or more fuel bundles 106.

The fuel assembly 106 can be any suitable nuclear fuel source,including, for example, a plurality of 50 cm long fuel bundles and/or aplurality of longer fuel bundles having a length between 3-6 m. The fuelassembly may include rods, pellets and any other fuel configuration.

In the present example, the reactor pressure vessel 104 is sub-dividedinto at least first and seconds portions or chambers, for example afirst or lower portion or calandria 105 a for containing the pressurizedheavy water moderator, and a second or upper portion 105 b that containsthe coolant and is generally separated from the moderator. The upperportion 105 b can define a plenum chamber for receiving and/or definingcoolant plenums within the reactor pressure vessel 104 (for exampleplenums 110, 112).

It is understood that a flow of coolant fluid, in a primary coolantloop, is circulated through the fuel channels 102 so that it can beheated by the energy release by the nuclear reactions in the fuelbundles/assemblies 106 and then used to produce steam and/or coolantheated to its supercritical state which drives turbines (not shown) forelectricity generation, and/or de-salination, and/or co-generation. Thefuel channels can form part of a larger, coolant treatment system orcoolant containment system as known in the art. In the present example,the moderator fluid is deuterium (which is also referred to as heavywater or D₂O) that is maintained at a first pressure within the pressurevessel. The coolant fluid can be heavy water or light water (H₂O) or anyother suitable coolant fluid known in the art. The fuel channels 102 aresealed within the reactor 100 so that the coolant in the fuel channels102 does not mix with the heavy water moderator contained in the reactorpressure vessel 104.

Conventional commercial pressure-tube type reactors can include aplurality of, horizontally oriented pressure tubes, each of which isconnected to a separate coolant inlet pipe or feeder and a separatecoolant outlet feeder. As known in the art, on-line refueling of somehorizontal commercial pressure tube type reactors is often done usingautomated refueling robots. The spacing or pitch between adjacent,horizontal pressure tubes can be determined primarily by physicsparameters but can also be influenced by the external pipingrequirements (for the inlet and outlet feeders) as well functionallimitations of the refueling robots (i.e., enough clearance must be leftbetween tubes to allow for the proper operation of the robots and forthe passage of feeder pipes), which may determine minimum dimensions.

The heavy water moderator in conventional pressure-tube reactors is heldat low pressure, i.e. less than 200 kPa. In such existing reactors, thecalandria need not be a thick-walled pressure vessel becausesubstantially all of the high-pressure aspects of the reactor arecontained by the pressure-tubes and the rest of the coolant containmentsystem.

In contrast, the reactor 100 is a pressure-tube type reactor but not allof the pressure load is carried by the pressure tubes under normaloperating conditions, and the heavy water moderator is pressurized andcontained within a suitable pressure vessel. Coolant flowing through thefuel channels remains isolated from, and does not directly mix with themoderator contained in the surrounding pressure vessel.

The provision of the moderator under pressure can reduce the pressuredifferential faced by the fuel channels by providing an intermediatepressure zone. In some examples, the moderator pressure can besubstantially the same as the coolant pressure in the fuel channels. Inother examples the moderator pressure can be different than the pressureof the coolant. Optionally, the moderator pressure can be between20%-110% of the coolant pressure. For example, if the coolant pressureis between 20-30 MPa, the moderator pressure can be between 5-25 MPa,and optionally 25-30 MPa or greater than 30 MPa.

An advantage of reducing the pressure across the fuel channels (e.g.across the pressure tubes described below) may be that the pressuretubes can be made correspondingly thinner. This enables the pressuretubes to be optimized with respect to the reactor physics. Additionally,for a reactor intended to operated in a supercritical regime, thatnecessarily requires high pressures, it enables these high pressure tobe present in the pressure tubes, without the pressure tubes requiringthick walls to safely withstand the internal pressure.

Optionally, the reactor 100 is vertically oriented, as illustrated, sothat the fuel channels 102 are arranged in a substantially verticalconfiguration, preferably so that a fuel channel axis 108 defined by thelongitudinal axis of any given fuel channel 102 is vertical (FIG. 2). Insome examples, the fuel channels 102 can be reentrant fuel channels (asdescribed below) so that coolant fluid can be supplied from the upperend of the reactor 100, at the inlet nozzle(s) 130, and withdrawn fromthe outlet nozzle(s) 131. In other examples, with different fuelchannels the coolant fluid can be supplied and withdrawn from differentareas or portions of the reactor 100.

In the described example, the plurality of inlet and outlet feedersrequired on existing pressure-tube type reactors are replaced with acoolant inlet plenum and a coolant outlet plenum. In this example, allof the fuel channels 102 in the reactor 100 are all supplied withcoolant from a single inlet header or plenum, for example inlet plenum110 that is connected to an inlet end of each fuel channel 102, and thecoolant exiting each fuel channel 102 (having been heated by the fuelbundles/assemblies 106) is collected in a single outlet header orplenum, for example outlet plenum 112 that is connected to an outlet endof each fuel channel. The reactor 100 cannot be refueled on-line becauseit is not possible to selectively open or access a portion of the fuelchannels 102 while leaving the remaining fuel channels 102 in operation.When the reactor 100 is taken off-line (i.e. shut down for maintenanceor refueling) the reactor 100 can be opened to allow servicing and batchre-fueling, as described in detail below (FIGS. 4 and 6).

The inlet plenum 110 includes a first or upper wall portion that formsan upper boundary on the plenum. In the present example, the upperboundary of the inlet plenum is defined by a lower surface of thereactor pressure vessel head 160. The inlet plenum 110 also includesside walls and second or lower wall portion. In the present example theinlet plenum side walls are provided by portions of the pressure vessel104, for example side walls 118, and the lower wall portion is providedby the upper surface of the fueling tubesheet 120. The external surface114 of the outlet plenum 112 provides the inside surface of the inletplenum 110. The outlet plenum external surface 114 separates the inletplenum 110 from the outlet plenum 112. Together, the upper wall, sidewalls, insulated lower wall, and outlet plenum external surfacecooperate to enclose an inlet plenum chamber 122 to receive coolant froma coolant supply source, for example, a coolant supply conduit connectedto the coolant containment system (not shown), and distribute thecoolant amongst the plurality of fuel channels 102. Coolant isintroduced into the inlet plenum 110 via one or more coolant inletports, for example coolant nozzles 130, that extend through the reactorpressure vessel side wall 118. In other examples, the coolant inlets canbe any suitable type of coupling, valve or connector known in the art.Optionally, the inlet plenum 110 can be supplied with coolant from oneor more coolant nozzles 130 depending on the desired coolant flow rateand/or other factors known in the art. Once in the reactor 100, thecoolant fluid follows a coolant flow path, schematically represent by aplurality of arrows 103

In the present example, the coolant enters the inlet plenum 110 as a lowtemperature subcritical fluid, at any suitable inlet temperature, forexample an inlet temperature between 280-350 degrees Celsius, and anysuitable inlet pressure, for example an inlet pressure between 15-30MPa. Optionally the coolant inlet pressure can be between 10-25 MPa. Thematerials used to form the inlet plenum 110 can be any suitable materialhaving the desired properties to withstand the expected coolant inletconditions, including, for example zirconium or stainless steel alloys.

In the present example, the outlet plenum 112 includes a first or upperwall, for example plenum cover 124, side walls, for example plenum sidewalls 126, and a second or lower wall, for example plenum tubesheet 116,that cooperate to define an outlet plenum chamber 128. The outlet plenumchamber 128 is configured to receive the coolant exiting each of thefuel channels 102 and direct the coolant downstream for furtherprocessing, including, for example steam generation, and/or processingin a nuclear turbine generator (not shown). Coolant exiting the fuelchannels 102 is collected in the outlet plenum chamber 128 and thenwithdrawn from the outlet plenum 112 and carried away for furtherprocessing (optionally including steam generation and/or processing in anuclear turbine generator) via one or more coolant outlet conduits (notshown). In the present example, the coolant outlet plenum 112 is fluidlyconnected to the coolant outlet conduits through coolant outlet nozzles131. The outlet plenum 112 can include any suitable number of outletnozzles 131. In the present example, the reactor 100 includes fouroutlet nozzles 131 spaced equally about the circumference of the outletplenum 112.

As illustrated, the outlet plenum side wall 126 is a single, annular orring-like member that is integrally forged with the plenum tubesheet116. In other examples, the side wall 126 can be formed from multiplepanels or segments, and can be separate from, but sealed to, the plenumtubesheet 116. In the illustrated examples, the plenum side wall 126includes four outlet nozzles 131, spaced equally around the perimeter ofthe plenum 112, for removing coolant from the chamber 128.

In other examples the reactor 100 can include a greater or fewer numberof outlet nozzles 131, and the outlet nozzles 131 may be arranged in anydesired configuration. Coolant outlet nozzles 131 can also extend acrossthe reactor pressure vessel pressure boundary, i.e. through a portion ofthe reactor pressure vessel walls, for example through calandria sidewalls 118. Coolant outlet nozzles 131 can be fluidly connected to, orcoupled with, any suitable coolant outlet conduits, for example pipes(not shown) to carry the coolant away from the reactor 100.

In some examples, the subcritical coolant exiting the fuel channels 102remains a subcritical fluid after being heated by the fuelbundles/assemblies 106. In other examples, the coolant exiting the fuelchannels 102 has been heated by the fuel bundles 106 to become asupercritical fluid, having an outlet temperature between 400-675degrees Celsius and an outlet pressure between 23-35 MPa (which may beslightly different than the inlet pressure due to flow losses and otherknown effects). The materials used to construct the outlet plenum 112,outlet nozzles 131 and at least the portions of the fuel channels 102exposed to the high temperature supercritical coolant can be selected towithstand the expected coolant conditions.

Exposure to flowing, high temperature supercritical fluids may causeaccelerated corrosion and surface wear on some materials. Referring toFIG. 5, in some examples, consumable, replaceable wear elements, forexample inserts or liners 134 can be inserted through the coolantoutlets 131 in the outlet plenum 112 to overlie the inner surfaces ofthe outlet nozzles 131, and optionally portions of the downstreamcoolant pipes 132, to prevent the high temperature supercritical coolantexiting the plenum 112 from contacting and damaging the surfaces of theoutlet nozzles 131 and pipes 132. In some instances it may be cheaperand/or simpler to replace a consumable liner 134 rather than having torefurbish or replace the outlet nozzles 131 or other portions of thehigh-pressure reactor pressure vessel 104. Liners 134 can be selectablywithdrawn and removed to enable inspection and maintenance on the outletnozzles 131 and to enable the outlet plenum 112 to be removed from thepressure vessel 104, as described in detail below. Optionally, theentire outlet plenum 112 can be designed as a consumable, replaceableelement that is intended to wear in order to preserve the integrity ofthe surrounding pressure vessel. It some instances it may be cheaper andeasier to provide multiple outlet plenums 112 in a nuclear power plantthan to repair or replace damaged portions of the thick-walled pressurevessel 104.

The inserts 134 can be coupled to the outlet plenum 112, outlet nozzles131 and pipes 132 using any suitable means, including bolts and weldedjoints. Referring to FIG. 5, in the illustrated example an adapter 133is provided to connect the outlet nozzle 131 and liner 134 to thecoolant pipe 132. In this example, the adapter 133 is welded to both theoutlet nozzle 131 and the pipe 132, providing a coolant tight connection(to inhibit leaks) that is capable of withstanding a supercriticalcoolant pressure. The insert 134 extends through the outlet nozzle 131and can be coupled to the outlet plenum side wall 126 and adapter 133using any suitable means, including bolts and gaskets (as shown,) weldedjoints (not shown) or the insert 134 can be threaded into the adapter133. The adapter 133 can be replaceable, so that the adapter 133 can bereplaced without requiring the replacement of the outlet nozzle 131 orother portions of the pressure vessel 104.

The insert 134 can be sized so that an annular space 135 is formedbetween the liner 134 and the outlet nozzle 131. The annular space 135can be in communication with the inlet plenum 110 so that relativelycooler coolant from the inlet plenum 110 can circulate between theinsert 134 and the outlet nozzle 131, in the annular space 135, toregulate the temperature of the outlet nozzle 131.

In some examples it may be desirable to inhibit heat transfer betweenthe incoming coolant fluid in the inlet plenum 110 and the alreadyheated coolant fluid in the outlet plenum 112. In such instances, tocontrol and/or inhibit heat transfer between the coolant held in theinlet plenum chamber 122 and the coolant held in the outlet plenumchamber 128 (and coolant in the portion of the fuel channels 102 thatextends through the inlet plenum chamber 122 as described below), thesurfaces separating the inlet and outlet plenum chambers 122, 128, forexample plenum tubesheet 116 can include a thermally insulatingmaterial. In some examples a separate thermal insulator can bepositioned between the inlet and outlet plenum chambers 122, 128. Thethermal insulator can be any suitable material, including, for exampleceramics and composite materials capable of withstanding hightemperatures. In other examples, insulating material may be incorporatedinto the materials used to form the plenum tubesheet 116 and fuelchannels 102. In some examples, insulating material can be incorporatedwithin portions of the plenums 110, 112, instead of providing a separateinsulator.

In some examples, the outlet plenum cover 124 can be a pressure bearingmember that is capable of withstanding the entire differential pressurebetween the outlet plenum chamber 128 and inlet plenum chamber 122. Inother examples, as illustrated, the reactor 100 can be configured sothat the reactor pressure vessel 104 forms a majority of the pressureboundary between the interior of the reactor 100 and the surroundings.

In the illustrated example, the reactor pressure vessel 104 includes agenerally cylindrical pressure bearing side wall 118 that is integrallyformed with a curved or dome-like bottom wall 158. A dome-likepressure-bearing reactor pressure vessel head or cover 160 is detachablyconnected to the side wall 118 using any suitable means, including, forexample, a plurality of bolts 162. The reactor pressure vessel head 160and reactor pressure vessel (body) 104) are sealed by mechanical gaskets(not shown). In other examples, the pressure vessel can be any desirableand suitable shape known in the art. The bottom wall 158 and/or thecover 160 can be flat or have any other desired shape. The lower portionor calandria 105 a, contains the moderator and is a distinct portion ofthe reactor pressure vessel 104. The calandria 105 a, is a chamberbounded by the lower surface of the fueling tubesheet 120, the reactorpressure vessel side wall 118, and the reactor pressure vessel bottomwall 158. The calandria 105 a, is penetrated by sealed pressure tubes138. The fueling tubesheet 120, the side wall 118, the bottom wall 158and pressure tubes 138, cooperate to enclose the calandria 105 a volume,in which the heavy water moderator is stored. The heavy water moderatorenters the calandria volume 105 a, via inlet nozzle(s) 161 in thereactor pressure vessel side wall. The lower support plate 168, providesradial support of the pressure tubes 138.

Coolant inlet nozzles 130, coolant outlet nozzles 131 and heavy watermoderator nozzles 161 (for supplying and/or removing heavy water to actas the moderator) can extend through portions of the side walls 118. Thecoolant fluid containment system and the moderator fluid containmentsystem (for supplying the heavy water moderator) can include anysuitable equipment known in the art, including for example pumps,storage tanks, accumulators, valves, heat exchangers and filters.

In the illustrated example, each fuel channel 102 is formed as areentrant fuel channel and includes an inner conduit that is receivedwithin a surrounding outer conduit. The inner conduit is configured toretain the fuel bundles, and the conduits are nested so that coolantfluid can flow through both the inner and outer conduits.

Referring to FIG. 2A an example of a fuel channel 102 for use in thereactor 100 is shown. The fuel channel 102 includes an inner conduit,for example inner fuel liner 136, that is received within acorresponding outer conduit, for example outer pressure tube 138. In thepresent example both the fuel liner 136 and pressure tube 138 aregenerally cylindrical, tube-like members and the fuel liner 136 isconcentrically aligned within the pressure tube 138 defining an annularspace 140 there between. The size of the annular space 140 is based onthe relative diameters 137, 139 of the fuel liner 136 and pressure tube138 respectively. Optionally, the fuel liner 136 may not beconcentrically positioned within the pressure tube 138, but may beoffset. In such examples the annular space 140 may have a width thatvaries around the circumference of fuel channel 102.

Coolant flow rate through the fuel channel 102 can be based on thecross-sectional area of the annular space 140. The relative sizes of thefuel liner 136 and surrounding pressure tube 138 can be selected toprovide an annular space 140 having a desired cross-sectional area.

In other examples, one or both of the fuel liner 136 and pressure tube138 can be non-cylindrical, provided that the fuel liner 136 can beadequately received within the pressure tube 138. In some examples atleast a portion of the walls of the fuel liner 136 can comprise aportion of the walls of the pressure tube 138 (i.e. a shared wallsegment).

A first end of each pressure tube 138, for example an upper, inletportion 144 of each pressure tube 138 is coupled to the fuelingtubesheet 120 and the annular space 140 is in fluid communication withthe inlet plenum chamber 122. In this example, the inlet portion 144 ofthe pressure tube 138 serves as the inlet for the corresponding fuelchannel 102. Each pressure tube 138 can be connected to the fuelingtubesheet 120 using any suitable connecting means known in the art,including rolled joints, welded joints, explosion bonding, etc. Theconnection mechanism can be selected based on the materials of thepressure tubes 138 and the fueling tubesheet 120.

Referring to FIG. 3, the number, configuration and arrangement or pitchspacing 141 of the apertures or openings 142 in the fueling tubesheet120 (defined as generally horizontal the distance between aperture axes143) can be any suitable distance and/or configuration known in the artto accommodate the desired number of pressure tubes 138 (and accordinglyfuel channels 102) in a given reactor. The arrangement of the latticespacing can be either rectangular/square or triangular/hexagonal ingeometry. In some examples the fueling tubesheet 120 is formed from thesame material as the pressure tubes 138. In other examples the fuelingtubesheet 120 and pressure tubes 138 are different materials.

The lower portion 146 of the pressure tube 138 is sealed in aliquid-tight manner, so that the interior of the pressure tube 138 (intowhich the fuel liner 136 extends) is separated from the pressurizedheavy water moderator surrounding the exterior of the pressure tube 138.In this configuration, any pressure differential between the coolantinsider the fuel channels and the heavy water moderator in the calandria105 a is carried by the pressure tube 138.

While the described calandria 105 a is a high-pressure vessel configuredto contain pressurized heavy water, in some examples there may be apressure difference between the coolant and the moderator. In someexamples, the heavy water within the calandria 105 a can be maintainedat a first pressure, for example between 5-25 MPa, optionally 15 MPa,and the pressure of the coolant flowing within the pressure tubes 138can be at a second pressure, for example between 20-35 MPa, optionally25 MPa. In the described example, each pressure tube 138 is capable ofwithstanding the resulting differential pressure of at least 10 MPa. Inother examples, heavy water in the calandria 105 a can be maintained atthe same pressure as the coolant flowing in the fuel channels 102. Insuch examples the pressure tube 138 need not be configured to withstanda substantial differential pressure. For any given reactor 100, thepressure tube 138 materials and wall thickness 148 can be selected basedon the expected pressure differential present within the reactor 100.

Pressure control systems are provided for both the coolant circulatingin the primary cooling loop and the moderator. Further, in someexamples, any suitable pressure relief device (such as rupture disk,pressure relief system, and both active and passive systems) may be usedto relieve abnormal pressure conditions. Rupture disks and the like haveparticular applicability to relieving pressure in accidental or abnormalsituations. The pressure boundary between the moderator and primarycoolant loop may be provided with rupture disks that vent the pressuretubes into the moderator, to cause rapid equalization in pressurebetween the primary cooling loop and the moderator; for supercriticaloperation, the quantity of fluid circulating in the primary cooling loopis not large, and it should be possible for some of its energy beabsorbed when in direct contact with the cooler moderator, in anemergency situation, without causing the moderator pressure to riseexcessively. Further the pressure control system for the moderator maythen be configured to handle a sudden influx of additional fluid fromsuch a pressure relief event, and/or the primary coolant system andmoderator system be coupled through a pressure equalizing system thatmaintains coolant separation.

Referring to FIG. 3, in this configuration, the reactor 100 comprisestwo separate, pressurized systems, separated by respective pressureboundaries. In the illustrated example, a first pressurized systemcontains the moderator at its first pressure. This first pressurizedsystem is defined by a first pressure boundary 172 that separates themoderator from the surrounding atmosphere and from the coolant flowingthrough the fuel channels 102. In the present example, the firstpressure boundary 172 is defined by the reactor pressure vessel walls118, 158, 160. In some examples the pressure in the calandria 105 a ofthe reactor pressure vessel 104 can be different than the pressure inthe upper portion 105 b. In such examples, the first pressure boundary172 may also include the fueling tubesheet 120.

The second pressurized system is contained within the first pressurizedsystem and, in the current examples, is defined by a second pressureboundary 174 that is formed by the walls of the inlet and outlet plenums110, 112 that contain the coolant fluid, including the fueling tubesheet120, and the walls of the pressure tubes 138. In this configuration, thefueling tubesheet 120 and pressure tubes 138 cooperate to maintain apressure difference between the moderator in the calandria 105 a of thepressure vessel 104 and the coolant fluid within the inlet plenum 110,upper portion 105 b and the fuel channels 102.

Both pressure boundaries are liquid-tight (except for intentional accesspoints and conduits) so that neither coolant fluid nor heavy water canpass through the first or second pressure boundary (unless the reactoris opened as described herein). It is understood that there may be localpressure variations within each pressurized system. For example, coolantpressure at the fuel channel outlets may be slightly lower than thepressure at the fuel channel inlets due to pressure losses experiencedby the coolant as it flows through the fuel channels.

Both the inlet portion and outlet portion of the fuel liner 136 are suchthat the fuel liner 136 forms a continuous fluid conduit for channelingcoolant from the inlet downward through the outer channel annulus space140, re-entering at the bottom of the fuel channel upward into thecentral fuel liner 136, past the fuel bundle/assembly 106, throughoutlet portion and into the outlet plenum chamber 128.

Referring to FIG. 2A, in the present example, the inlet portion of thefuel liner 136 is provided by spacing the open, lower end 150 of thefuel channel 102 apart from the sealed lower portion 146 of the pressuretube 138 by a pre-determined distance 154. The magnitude of thisdistance 154 can influence the flow of the coolant through the fuelchannel 102, and can be selected to provide the desired flowcharacteristics. In the present example, the distance 154 is chosen tooptimize the coolant flow characteristics.

In other examples, the lower end 150 of the fuel liner 136 can be sealedand/or connected to the pressure tube 138, and the inlet portion of thefuel liner 136 can comprise one or more apertures formed in, andextending through the walls of the fuel liner 136 (not shown).

In any example, the lower portion 146 of the pressure tube 138 and theinlet portion of the fuel liner 136 can be of any suitable shape orconfiguration to provide the desired coolant flow conditions within thefuel liner 136, including, for example, laminar flow, turbulent flow,rotational or vortex type coolant flow around the fuel bundle/assembly106. Additionally the lower end 150, can have features (not shown) tocontrol the flow in the channel (i.e. a nozzle).

The outlet portion of the fuel liner 136 is provided by the open, upperend 152 that is coupled to the outlet plenum tubesheet 116, so thatcoolant flowing out of the upper end 152 of the fuel liner 136 entersthe outlet plenum chamber 128. The upper end 152 of the fuel liner 136can be coupled to the plenum tubesheet 116 in any suitable manner, asexplained above. In this example, the upper end 152 of the fuel liner136 provides the outlet of the fuel channel 102.

In the present example, the fuel liners 136 are not directly coupled tothe pressure tubes 138 so that the fuel liners 136 can be freely removedfrom the pressure tubes 138 when desired, and may be considered asconsumable. In this configuration, the entire outlet plenum 112 isdetachably connected to the reactor 100 (using any suitable method knownin the art including bolts) so that the outlet plenum 112 and the fuelliners 136 coupled to the plenum tubesheet 116 can be separated from therest of the reactor 100 (as described in detail below) as a single unitor sub-unit, for example as a core module 156, for example as shown inFIG. 4 and as illustrated schematically in FIG. 6. When the outletplenum 112 and fuel liners 136 are removed the fueling tubesheet 120 andpressure tubes 138 can remain in place, thereby containing thepressurized heavy water moderator in the calandria 105 a in the reactorpressure vessel 104. In such configurations, the pressure tubes 138 andfueling tubesheet 120 are sized to withstand the expected operatingpressure differential and at a maximum the depressurized moderator orprimary heat transport system differential state.

Referring to FIG. 6, in some instances, operators of the reactor 100 canhave one or more extra or replacement core modules 156 b that arecompatible with the reactor 100. The ability to remove the outlet plenum112 and fuel liners 136 (optionally containing the spent fuel bundles)as a single core module 156 enables batch refueling of the reactor 100,in which, optionally, a core module containing spent fuelbundles/assemblies 106, for example a used core module 156 a, can beswapped with a replacement core module 156 b that contains new fuelbundles/assemblies 106. Swapping complete core modules 156, as opposedto individually swapping the fuel bundles/assemblies 106 in each fuelliner 136, may speed up the refueling process and may reduce reactordowntime. When a core module 156 is removed from the reactor 100 it maybe inspected, serviced, re-fueled, refurbished or disposed of, asnecessary.

In the present example, one example of a batch-refueling processincludes the steps of detaching used core module 156 a from the pressurevessel (or any other portion of the reactor 100) to simultaneouslyextract the outlet plenum 112 and the plurality of fuel liners 136 fromthe reactor 100, thereby simultaneously extracting the plurality of fuelbundles retained within the fuel liners 136.

Once the used core module 156 has been extracted, a replacement coremodule 156 b can be inserted into the reactor 100. That is, core modules156 a, 156 b can be swapped or exchanged.

Optionally, a containment pool 164 can be provided to store replacementcore modules 156 b to be inserted into and coupled to the reactor 100and optionally, to receive the used or spent core modules 156 aextracted from the reactor 100. Coupling the second, replacement coremodule 156 b to the reactor 100 includes the steps of aligning each ofthe plurality of fuel liners 136 with their corresponding pressure tubes138 that remained attached to the reactor pressure vessel 104. Onceproperly aligned, the replacement core module 156 b can be lowered toposition the fuel liners 136 in their operating positions in which theyare at least partially received in their corresponding pressure tubes138. Once properly inserted, the replacement core module 156 b can becoupled to the reactor pressure vessel 104 (or any other suitablemember), the reactor pressure vessel head 160 can be re-attached to theside walls 118 and the reactor 100 can be restarted.

Another example of batch-refueling comprises the steps of opening thepressure vessel, by removing the cover 160, but leaving the core module156 within the reactor 100. Instead of removing the core module 156, anoperator can detach the outlet plenum cover 124 from the outlet plenumsidewalls 126 to provide access to the outlet plenum chamber 128 and theplurality of fuel liners 136. In this example, spent fuelbundles/assemblies 106 can be removed from the fuel holder apparatus(not shown) within each of the fuel liners 136 using an overhead craneor any other suitable apparatus known in the art. Optionally, the fuelbundles/assemblies 106 can be removed from some or all of the fuelliners 136 simultaneously. Once emptied, fuel liners 136 can bere-fuelled by inserting new fuel bundles/assemblies 106 into fuelholders of the existing fuel liners 136. In this example the core module156 can be re-used. This enables a single core module 156 to be used formultiple reactor cycles (i.e. the period between start-up and shutdownduring which the reactor 100 is used to generate power).

In some examples, a reactor 100 can be re-fueled using either or both ofthe methods described above.

In any of the examples described herein, the fuel channels 102,including both the fuel liners 136 and the pressure tubes 138, can beformed from any suitable material that has the desired mechanicalproperties and has sufficiently high neutron transmissibility to enablethe desired nuclear fission reaction within reactor 100, as known in theart. In some examples, both the fuel liners 136 and the pressure tubes138 are formed from zirconium alloys known in the art to besubstantially transparent to neutrons generated during the nuclearreaction. In other examples, the pressure tubes 138 are formed from azirconium alloy and the fuel liners 136 are formed from a stainlesssteel alloy to withstand exposure to coolant in a supercritical state.

Because the pressure tubes 138 are sized to withstand substantially allof the pressure differential between the coolant and the heavy watermoderator, the pressure differential across the walls of the fuel liners136 (for example caused by the different flow velocities and boundarylayer effects experienced by the coolant in the annular space 140 andthe coolant within the fuel liner 136) can be relatively small, forexample less than 1 MPa, which can enable the fuel liners 136 to berelatively thin walled (compared to the pressure tubes 138). Providingthin walled fuel liners 136 enables the fuel liners 136 to be formedfrom the desired stainless steel alloy while still remainingsufficiently transparent to neutrons.

The pressure tubes 138, and/or the fuel liners 136, can be formed fromany suitable material. The material selected can be chosen based on aplurality of factors, including, for example, optimization propertiesfor neutron absorption, strength, corrosion resistance, creepresistance, fracture toughness and temperature resistance. Optionally,the pressure tubes 138, can be formed from a material that has a neutronabsorption cross-section between 150-300 mb. In some instances, the fuelliners 136 can formed as thin walled tubes so that a desired neuronabsorption (i.e. allowing the passage of a sufficient number ofneurtons) can be maintained despite the fuel liners 136 being made of amaterial having a neutron absorption cross-section of 3-4 barns.

In addition to containing and routing coolant, some or all of the cover160, plenum cover 124, side walls 126, plenum tubesheet 116 and thevolume of coolant retained within the both the outlet plenum chamber 128and inlet plenum chamber 122 can provide radiation shielding at the topof the reactor 100. In some examples, some of all of these elements canprovide a sufficient or desired level of radiation shielding so that thereactor 100 does not require a separate upper shield member. In otherexamples, the reactor 100 can include a separate upper shield, forexample a neutron shield as known in the art (not shown), to provide adesired or required level of radiation shielding toward the top of thereactor 100. The radiation end shield can be any radiation shieldapparatus known in the art, including the neutron reflector thatincludes an outer shell filled with spherical steel balls. A separateshield, if desired, can be located in any suitable location as known inthe art, including, for example, between the plenum cover 124 and thecalandria cover 160 and above/surrounding the calandria cover 160.Optionally, radiation shielding can also be provided around the sidewalls 118 and bottom wall of 158 of the pressure vessel 104.

The fuel channels 102 are disposed in a vertical orientation, as inreactor 100, thermal expansion and radiation creep generally can resultin an axial lengthening of the fuel liners 136 and/or pressure tubes138. In this configuration, changes in fuel liner 136 and pressure tube138 lengths will generally not generally affect the radial spacing, i.e.the size of the annular spacing, between the fuel liner 136 and pressuretube 138.

In some examples, the expansion, or growth, of the fuel channels 102(i.e. fuel liner 136 and pressure tube 138) may not be consistent oruniform across the reactor 100. For example, local differences inoperating temperature, radiation flux, fuel bundle condition, coolantand/or moderator pressure and other factors can lead to differentialgrowth of the fuel channels 102. That is, some fuel liners 136 andpressure tubes 138 can grow or lengthen by a greater or lesser amountthan other fuel liners 136 and pressure tubes 138 in the same reactor100. To account for the thermal expansion and creep of the fuel liners136 and pressure tubes 138 described above, the pressure tubes 138 maybe freely mounted a lower plate 168 of the pressure vessel 104, so as tobe able to move axially (vertically as shown) relative to the plate 168while helping to maintain desired fuel channel spacing or pitch.

As shown, the plate 168 can have non-circular openings 170, e.g. squareor rectangular openings, for the pressure tubes 138 to permit freemovement of the pressure tubes 138, and to equalize pressure on eitherside of the lower plate 168. In other examples the openings 170 can becircular, triangular or any other suitable shape and can be sized tofreely receive the pressure tubes 138 while still allowing a sufficientpassage of heavy water to balance the pressures on either side of theplate 168.

In some examples, instead of a continuous plate that includes apertures170, the lower support plate 168 can be a lattice of cross-members (flatmembers or circular rods) or any other suitable structure. The supportplate 168 can also include apertures 170 (or gaps in a latticeconstruction) to receive other reactor components, including controlrods and the like.

In some examples the coolant fluid supplied to the reactor 100 (i.e.pumped into inlet plenum 110 and circulated through pressure tubes 138)can be at generally the same temperature as the heavy water moderatorcontained in the reactor pressure vessel 104. In other examples, theincoming coolant fluid may be warmer or cooler than the heavy watermoderator.

In some examples, the pressure tubes 138 can be provided with additionalinsulating material to reduce heat transfer between the coolant fluid inthe pressure tubes 138 and the heavy water moderator contained in thelower portion 105 a. Optionally an insulator, for example a ceramicinsulating sleeve, can be provided to surround the inner or outersurfaces of the pressure tube 138 (not shown).

In the present examples, the interior of the inlet plenum 110, forexample chambers 122, is configured as a continuous, open cavity. In theabove examples, the coolant fluid will tend to divide amongst the fuelchannel inlets, i.e. pressure tubes 138, because the pressure losses inthe pressure tubes 138 are significantly higher than the pressure lossesin the inlet plenum chamber 122. The coolant flow rate through each ofthe pressure tubes 138 is controlled by features (not shown) in the fuelliner bottom 150, that limit the flow as required to remove theappropriate amount of heat from each fuel channel. In some examples, thecoolant flow rate through fuel channels 102 located toward the centre ofthe calandria 105 a is higher than the coolant flow rate through fuelchannels 102 located toward the periphery of the calandria 105 a.

While the fuel channels 102, and both the inner and outer conduitstherein, in the reactor 100 are described and illustrated assubstantially cylindrical or pipe-like members, it is understood thatthe fuel channel conduits can be of any suitable, complimentarycross-sectional shape and configuration known in the art, including, forexample, oval, arcuate, polygonal and rectangular cross-sectionalshapes.

While not described in detail, it is understood that the reactor 100 caninclude any known reactivity mechanisms (both in and out of the reactorcore), reactor control devices and reactor safety devices known in theart, for example as used with existing heavy water moderatedpressure-tube type reactors such as CANDU® reactors. Such devices caninclude, for example, control rods, liquid neutron poisons, shut offrods, liquid zone controllers, etc.

It is understood that fail-safe control rods (not shown) are one exampleof a reactor shutdown system that is configured to rapidly andautomatically terminate reactor operation. Control rods can introducenegative reactivity by absorbing excess neutrons when inserted betweenpressure tubes. In the example illustrated, the control rods areinserted from beneath the calandria 116, and can be inserted through thebottom wall.

Optionally, the control rods penetrate the calanria vessel 116 at anangle and operate on a fail-safe principle such that, in the event of anemergency reactor trip, the clutches that keep each control rod in itsstorage position are de-energized causing the control rods to beinserted or dropped into the calandria vessel 116 under the force ofgravity. In some examples, the reactor 100 can be controlled by one ormore various reactivity control devices including liquid zonecontrollers, adjuster rods and absorber rods.

One example of a liquid zone controller includes a plurality of fixedcontrol rods with controllable light-water filled compartments.Optionally, the liquid zone controllers can be positioned horizontally,penetrating the calandria vessel 116 in a horizontal plane. By changingthe level of H₂O in individual compartments, reactivity of the core canbe changed locally.

Optionally, adjuster rods (which are normally inserted fully in thecore) can be partially moved out to change reactivity. The adjuster rodscan extend horizontally.

Optionally the absorber rods can be similar to the fail-safe controlrods, and can be used for fast power reduction. The absorber rods can beconfigured to be gravity fed, in the same manner described above, andhence, they can oriented at an angle from the vertical.

A liquid neutron poison can be inserted using an active or passivesystem or structure, during abnormal or accidental situations, or for aguaranteed shut down. It may be contained within a housing with arupture disk or containment, within the calandria, configured toautomatically rupture if an abnormal pressure condition is detected. Itcan also be configured to rupture if an abnormal pressure condition isdetected in the primary cooling loop. The neutron poison may comprisegadolinium nitrate or boric acid or any other suitable neutron poisonknown to the art.

What has been described above has been intended to be illustrative ofthe invention and non-limiting and it will be understood by personsskilled in the art that other variants and modifications may be madewithout departing from the scope of the invention as defined in theclaims appended hereto.

The invention claimed is:
 1. A nuclear reactor comprising: a) a pressurevessel for containing a pressurized moderator at a first pressure thatis between about 5 MPa and about 25 MPa; b) a coolant inlet plenumdisposed at one end of the pressure vessel and having at least one inletport for receiving a coolant fluid, the coolant inlet plenum comprisingan inlet plenum upper wall, an inlet plenum lower wall spaced apart fromthe inlet plenum upper wall in a first direction and an inlet plenumsidewall extending therebetween; c) a coolant outlet plenum disposedwithin the coolant inlet plenum, the coolant outlet plenum comprising anoutlet plenum upper wall, an outlet plenum lower wall spaced apart fromthe outlet plenum upper wall and an outlet plenum sidewall extendingtherebetween, and wherein the outlet plenum lower wall is spaced betweenthe inlet plenum lower wall and the outlet plenum upper wall in thefirst direction; d) a plurality of fuel channels for a coolant fluid ata second pressure and fluidly connected at inlet ends thereof to thecoolant inlet plenum, the fuel channels adapted to receive nuclear fuelbundles and to be mounted within the pressure vessel and surrounded bythe moderator, and outlet ends thereof being fluidly connected to thecoolant outlet plenum to enable the coolant fluid to circulate from thecoolant inlet plenum through the fuel channels to the coolant outletplenum; e) the plurality of fuel channels maintaining separation betweenthe coolant fluid circulating within the fuel channels and themoderator.
 2. The nuclear reactor of claim 1, wherein the coolant inletplenum is separated from the moderator by a fueling tubesheet and theinlet ends of the fuel channels are coupled to the fueling tubesheet,wherein the moderator is maintained at the first pressure and thecoolant fluid is maintained at the second pressure higher than the firstpressure, and wherein the fueling tubesheet and the fuel channelsmaintain the pressure difference between the first and second pressures.3. The nuclear reactor as claimed in claim 2, wherein the pressurevessel comprises the inlet plenum upper wall and the inlet plenum sidewall and the pressure vessel is adapted to withstand the first pressure,and the fueling tubesheet and the fuel channels are adapted to withstandthe difference between the first and second pressures.
 4. The nuclearreactor of claim 1, wherein each fuel channel is a reentrant fuelchannel that comprises an inner conduit received within a correspondingouter conduit, wherein each outer conduit comprises the inlet end of therespective fuel channel, each inner conduit comprises the outlet end ofthe respective fuel channel to enable the coolant fluid to circulatethrough both the outer conduits and inner conduits, and wherein eachinner conduit is sized to both retain a nuclear fuel bundle and to bereceived within the corresponding outer conduit spaced therefrom toenable the coolant fluid to flow between the inner conduit and thecorresponding outer conduit.
 5. The nuclear reactor of claim 4, whereinthe plurality of inner conduits are removably received within thecorresponding outer conduits.
 6. The nuclear reactor of claim 5, whereinthe inlet plenum upper wall comprises an openable inlet plenum cover andthe coolant outlet plenum is detachably mounted within the coolant inletplenum and is removable from the coolant inlet plenum when the inletplenum cover is open, and removing the coolant outlet plenum causes eachinner conduit to be removed from the corresponding outer conduits, thecoolant outlet plenum and plurality of inner conduits defining a firstcore module.
 7. The nuclear reactor of claim 6, further comprising atleast a second coolant outlet plenum coupled to at least a secondplurality of inner conduits defining at least a second core module, theat least a second core module being exchangeable with the first coremodule.
 8. The nuclear reactor of claim 4, wherein each outer conduithas an open first end, comprising the fuel channel inlet end, connectedto the coolant inlet plenum to receive the coolant fluid, and a sealedsecond end to channel coolant fluid from the outer conduit into an openfirst end of the corresponding-inner conduit.
 9. The nuclear reactor ofclaim 1, wherein the pressure vessel comprises at least one vessel sidewall and a vessel cover detachably connected to the at least one sidewall, the vessel side wall and vessel cover cooperate to encase thecoolant outlet plenum, the coolant outlet plenum being accessible whenthe vessel cover is detached from the vessel side wall.
 10. The nuclearreactor of claim 9, wherein the outlet plenum upper wall furthercomprises a detachable outlet plenum cover to allow simultaneous accessto an outlet plenum chamber and the respective outlet ends of theplurality of fuel channels.
 11. The nuclear reactor of claim 1, whereinthe second pressure is substantially the same as the first pressure. 12.The nuclear reactor of claim 1, wherein the second pressure is greaterthan the first pressure.
 13. The nuclear reactor of claim 1, wherein thefirst pressure is between 20%-110% of the second pressure.
 14. Thenuclear reactor of claim 1, wherein the second pressure is between 20and 30 MPa.
 15. The nuclear reactor of claim 4, wherein each outerconduit has a neutron absorption cross-section between 150-300 mb. 16.The nuclear reactor of claim 4, wherein the plurality of inner conduitsare formed from a different material than the plurality of outerconduits.
 17. The nuclear reactor of claim 1, wherein the inlet ends ofthe plurality of fuel channels are adapted to withstand subcriticalcoolant fluid and the outlet ends of the plurality of fuel channels areadapted to withstand supercritical coolant fluid.
 18. The nuclearreactor of claim 1, wherein the moderator is heavy water and the coolantfluid is at least one of heavy water, and light water and gas.
 19. Thenuclear reactor of claim 4, wherein each outer conduit can withstand apressure difference between 1 and 25 MPa, and each inner conduit canonly withstand a pressure difference of less than 5 MPa, during normaloperating conditions.
 20. The nuclear reactor of claim 10, wherein theoutlet plenum cover is accessible when the vessel cover is detached fromthe vessel side wall and is covered by the vessel cover when the vesselcover is attached to the vessel side wall.
 21. The nuclear reactor ofclaim 1, wherein the coolant inlet plenum comprises at least one outletnozzle and the coolant outlet plenum comprises at least one coolantoutlet port fluidly connected to the outlet nozzle by a connectingconduit that extends through the coolant inlet plenum whereby coolantfluid can be conveyed from the coolant outlet plenum to a downstreamcoolant conduit that is external to the pressure vessel.
 22. The nuclearreactor of claim 21, wherein the connecting conduit comprises areplaceable liner that extends within the coolant outlet port and is indirect physical contact with the coolant fluid within the coolant inletplenum.
 23. The nuclear reactor of claim 1, wherein the outlet plenumsidewall is generally cylindrical and the inlet plenum sidewall isgenerally cylindrical and surrounds and is spaced apart from the outletplenum sidewall.
 24. The nuclear reactor of claim 1, wherein the coolantoutlet plenum upper wall is spaced below the inlet plenum upper wall inthe first direction.